1. Field of the Invention
This invention relates to a method and circuit for controlling the flow rate of the hydraulic oil in the brake cooling system of a vehicle.
2. Description of the Prior Art
Some types of vehicles, especially, but not exclusively, industrial or commercial vehicles, are required to work in extremely low outdoor temperatures, where they may also be parked with the engine off for considerable lengths of time; this may be the case, for example, in arctic environments or wherever the temperature falls well below zero at night. In such conditions, when these vehicles are (re)started it takes them a long time to reach the steady state, especially as regards the temperature of the fluids, such as the engine cooling water, engine lubricating oil, transmission oil and the oil of the brake cooling system, in vehicles equipped with oil-immersed brakes.
In some industrial applications where the costs involved are particularly high and vehicles are required to work in extremely harsh environmental conditions, the braking system is housed in a sealed oil bath to guarantee high braking torques while enhancing the vehicle's service life.
Two fluids can flow through the braking chambers in different circuits:                the brake fluid, that flows under high pressure, up to and above 100 bar;        the brake cooling oil, that is separate from the brake fluid and ideally requires minimum backpressure levels (usually between 2 and 6 bar) which are strongly influenced by the operating temperature. The backpressure threshold depends on the type of seal that is used and prevents the risk of cross-contamination between the two oils. It is easy to maintain low backpressure levels under normal working conditions, with an oil temperature of approximately 80° C., but this becomes more of a problem in cold start conditions, with outdoor temperatures of less than 5° C., due to the significant increase in the viscosity of the oil. In order to overcome this drawback the flow rate of the cooling oil must be reduced until this has warmed up sufficiently. During this time the brakes are not fully operational, in that only a part of the oil that is available is circulating. In this phase excessive use of the brakes could cause irreversible damage.        
Hence the need to optimize the method for controlling the oil flow rate during the time required for the hydraulic oil in the brake cooling system to warm up, in order to increase reliability and safety.